The present invention relates to a digitiser for use in a position sensor and in particular, to a digitiser for use in a capacitive touch screen.
Computer devices are well known and significant research has been made into man machine interfaces that allow humans to interact with computer devices. For example, capacitive touch pads are well known and allow a user to move a cursor around the screen by moving their finger over the touch pad. Touch screens are also well known in which an X-Y digitizer is mounted on or under the surface of a computer display and which allows the user to make selections directly on the display using either their finger or a conductive or an electromagnetic stylus. One type of digitiser commonly used in such touch screens has a grid of conductors arranged in perpendicular directions over the display screen and electronics arranged to measure the change in mutual capacitance at each intersection point formed by the crossing X-Y conductors as a finger (and/or stylus) moves over the screen. Typically, the grid pitch (centre to centre distance between adjacent conductors) is between 5 mm and 20 mm, which provides a sensing resolution that is sufficient to detect a human finger anywhere over the grid. In many applications, the conductors are formed from indium tin oxide (ITO) as these conductors are transparent. Ultra-thin copper wire is also commonly used to form the conductors of X-Y digitisers, although some users complain about being able to see the copper traces when used in smaller display screens.
Such a design of X-Y conductors is illustrated in FIG. 1a. As shown, in FIG. 1b, when an excitation voltage is applied to an X conductor, it generates an electric field that couples with a Y conductor at the intersection point between the X-Y conductor pair. The amount of coupling defines the mutual capacitance between the two conductors. When a finger (or conductive stylus) is present over or near this intersection point, as shown in FIG. 1c, some of the generated electric field couples into the finger and thereby reduces the coupling (and hence mutual capacitance) between the X-Y conductor pair. Thus electronics coupled to the grid of conductors can sense the change of mutual capacitance and thereby the presence and location of the finger over the grid.
When designing such X-Y digitisers for touch screen applications, there are a number of design challenges and tradeoffs. One challenge is that the mutual capacitance between each X-Y pair of conductors is relatively small and the change in mutual capacitance due to the presence of the finger (or stylus) is smaller still. As a result, the measurements are often swamped by other signals, such as by switching noise associated with the switching of the LCD panel over which the X-Y conductors are placed and capacitive cross-talk between adjacent conductors of the X-Y grid. The digitiser has to be designed so that the measurements can be reliably performed at a high enough update rate to support a natural drawing experience with the user's finger or stylus. The digitiser must do this whilst using low cost electronics in order to provide a low cost system for the consumer product market.
One of the major sources of unwanted error in the measurement signals is capacitive cross-talk between adjacent grid conductors. In particular, when an excitation signal is applied to one excitation conductor, that excitation signal capacitively couples with adjacent excitation conductors. Similarly, signals coupled into one detection conductor will capacitively couple into the adjacent detection conductors as well. This can lead to significant cross-talk error in the signals being measured. This cross-talk can be minimised by keeping the frequency of the excitation signal that is applied to the excitation conductor as low as possible. However, if the excitation frequency is too low, then it becomes difficult, or more expensive, to achieve the desired measurement update rate that will allow the tracking of the user's finger or stylus over time.
Further, the problems of measurement update rates and cross-talk get worse as the size of the grid increases. This is because with a larger grid, there will be more X-Y conductors (to achieve the same spatial resolution) and hence more intersection points to measure; and as each grid conductor becomes longer, its distributed resistance gets larger and the distributed mutual capacitance between adjacent wires gets larger, which in turn increases the cross-talk error as a square of the diagonal size. Thus as the size of the grid gets larger, digitisers formed using ITO conductors face greater design challenges than those that use metallic based conductors (such as copper) because ITO has a much higher resistance than a metal conductors.
A further problem with increasing size relates to the number of measurement channels that are used. In particular, in most digitiser systems, the signals from the detection conductors are multiplexed through a number (sometimes one) of separate measurement channels before being processed by a microprocessor. However, as the display gets larger, the number of X-Y conductors increases to keep the same spatial resolution. Thus more measurements have to be made and this can result in the need to increase the number of measurement channels. However, this increases the cost of the digitiser.
FIG. 2 is a plot that shows how the power consumption and the cost of an ITO based digitiser and of a metallic conductor based digitiser increases with the size of the display. As shown, for small sized displays (diagonal size less than about 40 cm), ITO provides the same performance as the metallic conductor based digitiser both in terms of power consumption and cost. A metallic conductor based digitiser of such sizes can provide an update frequency well above the nominal 100 Hz, but such performance is not required for man machine interfaces. However, with ITO based systems, the power consumption and the cost increase exponentially with increasing display size. This is because, the lower excitation frequency required by the ITO based digitiser (to minimise capacitive cross-talk error) means that more measurement channels are needed to process the signals from the grid of conductors compared with a metallic conductor based digitiser. In particular, as metallic conductor based digitisers can use higher excitation frequencies, there is more time to multiplex the signals from more detection conductors through each measurement channel—which helps to keep the cost and power consumption down. As shown in FIG. 2, the thin metallic conductor based digitiser can be implemented with a relatively modest increase in the cost and power consumption for displays having a diagonal size up to 200 cm (80″). The step change in both the cost and power consumption caused by the requirement of adding even more measurement channels become evident for metallic wire digitisers having a diagonal size above 250 cm.
Another challenge with large scale X-Y digitiser systems is the desire for the system to be able to measure simultaneously a large number of independent touches. For large diagonal displays (over 100 cm diagonal), the system might have to be able to detect over 10 different touches in order to enable a true multi-user interaction. This places further constraints on the design of the digitiser.
The inventor has designed a number of new digitisers (and parts thereof) that try to address one or more of the challenges and conflicting requirements described above. In so doing, the inventor has made a number of different inventions that are described and some of which are claimed herein. The new designs of digitiser can be used in touch screens or in separate touch pads/whiteboards. The digitisers that are described are ideally suited for use with large scale display screens having diagonal sizes greater than, for example, 38 cm (15″) due to the ability of the algorithm to scale to large X-Y grids. For instance it allows to maintain an update rate of about 100 Hz whilst measuring the mutual capacitance at each node of a 320×180 X-Y grid. The application also describes a number of ways in which the grid of X-Y conductors and associated electronics can be manufactured and assembled.